Thermionic cathode



A ril 6, 1937.

E. Y. ROBINSON C CATHODE THERMIONI Filed June 24, 1927 2 Sheets-Sheet 1' iltorizey.

April 1937- .E. Y. ROBINSON v 2,075,910

THERMIONIC CATHODE Filed June 24, 1927 2 Sheets-Sheet 2 ha/e17! Zzvzed? 205 920022 Patented Apr. 6, 1937 UNITED STATES PATENT. OFFICE signor to Associated Electrical Industries Limited, a company of Great Britain Application June 24, 1927, Serial No. 201,184 In Great Britain July 7, 1926 13 Claims.

This invention relates to thermionic electrodes such as cathodes of the kind generally known as equipotential cathodes, in which an electron emitting member is heated by heat radiation and/or conduction from a heating member which is itself adapted to be heated electrically.

It has heretofore been proposed to construct cathodes by insulating heating members such as filaments of tungsten for example by members such as quartz or zircon tubes into which the filament is inserted and to place such insulating member within a metal tube or casing on the surface of which an electron emitting coating is applied. To obtain the best results with such a cathode and to obtain a thermionic valve having a low impedance, the cross section of the insulator should be small so that the ratio of cathode surface to the surface of the heating member is as small as possible, because with an oxide coated cathode which is in general operated at a temperature in the neighbourhood of 900 C. the heating member must operate at a temperature higher by an amount depending upon the 5 ratio of cathode surface to surface of the heating member. If the temperature of the heating member is high troubles arisefor instance by chemical reaction between the heating member and insulator and decomposition of the insulator,

30 thereby spoiling the vacuum in the device in which the cathode is used. The heating member moreover, should be as good a fit as possible within the insulator in order to increase the heat transferred to the cathode by conduction. 35 Heretofore it has been the practice to make the insulator and heating member at least as separate units and then to assemble the heating member within the insulator. It has been found difiicult to make small insulator tubes of exter- 40 nal diameter of less than about 1 mm.

According to the invention the heating member is enclosed where necessary by refractory insulating material by moulding said material around it or by dipping, spraying or otherwise 45 coating said member with a suspension, sludge or paint consisting of an insulating substance and a vehicle. The insulating material may then be baked, sintered or even fused on to the heating member. The insulated heating mem- 50 ber thus prepared may then be enclosed in a metal casing having a surface which is adapted to emit electrons when heated, such as a surface coated with a mixture of alkaline earth oxides. I ,Preferably the metal casing is deposited on' to.

the surface of the insulator by electric deposi tion or by spraying or by chemical deposition, or a combination .of these methods. The insulated heating member may however be enclosed in a separately formed metal tube. V

For an equipo-tential cathode of the kind re- 5 ferred to, it is desirable that the insulator consist of a material which, under the operative conditions of the cathode, particularly as to temperature, will retain its insulating properties and will not undesirably react with the heater ma- 10 terial during a reasonably long life. For the insulator itself kaolin and its associates is preferred. Other substances may be added such as felspar, lime or the like to facilitate sintering and 1 to give a stronger body. Clay slip may con- 15 veniently be used. The addition of the other substances is not essential because the process permits the employment of an insulator in a very friable form because after the insulator is once enclosed within the metal tube, even if 20 chipping or splitting of the insulator occurs, no damage will usually result. I 1

In one method of carrying out the invention a straight tungsten or molybdenum filament of 0.1 mm. diameter is drawn through a paste or cream formed of kaolin and water. The coating thus produced upon the filament is dried, preferably by natural evaporation in the atmosphere and is then given a preliminary baking in a muffle at a temperature of about 800 C. to effect further drying. Alternatively, the coated filament may be passed directly into the muflle and dried therein without previous natural drying, this latter procedure being frequently more convenient for mass production. Or the coating may be dried entirely naturally or in a gas flame. As many coatings as are necessary to give the required thickness of deposit may be applied in this manner, two coatings generally being sufficient. The coated heating unit is then preferably heated to a high temperature in an oven or gas flame to fire the kaolin, preferably so that the kaolin is completely vitrified. The time of firing the coating is preferably such that the material is vitrified but not sufficient for the tungsten crystals to be equiaxed and the filament thus rendered brittle. For this reason flame firing is in general preferable since the operation can be carried out more rapidly. Alternatively the coated heating unit may be fired by passing a current through the heater this being effected in hydrogen or in vacuum. A temperature of g from 1450" C. to 1500 C. has been found satisfactory but the length of time and also the temperature depends upon the quality of the kaolin employed and the impurities therein.

After firing, the heater unit is ready for the provision of the co-operating equipotential surface which conveniently surrounds or encloses the heater and may consist of a metal tube mounted thereon or a metal surface adhering thereon. The metal surface may be produced by first coating the insulator with platinum by dipping the insulator into a solution of platinum chloride, and then heating it so that platinum is deposited. A number of coatings, for example six, may be applied'in this manner. Alternatively a metal coat may be sprayed on by the Schoop method. In the latter case the coating may conveniently consist of nickel. The metal coating may be applied by other chemical methods or by an electrical deposition process. The member thus formed may now if desired be nickel plated until the required thickness of metal coating is obtained and then an oxide coat may be applied to the surface in the usual manner. One process of applying the oxide coat consists in dipping the cathode into or painting it with a suspension of barium and strontium carbonates in water and then heating it in a carbon dioxide atmosphere, the carbonate coating thus applied being subsequently converted to oxide.

By applying the metal coating in the manner 3 described above the thickness of the cathode can be made very small. The insulated heater may however be enclosed in a metal tube which may be formed separate from the heater and the insulated heater then slipped in, or on the other hand the tube may be formed from thin sheet wrapped or formed round the insulated heater. In one modification the cathode consists of a small nickel tube of oval cross section, into which is slid an insulated hairpin heater. In another '40 modification nickel or platinum foil may be formed round the insulated heater or platinum strip may be wrapped round the insulated heater, so as to cover its surface.

The cathode resulting consists of a heating member of tungsten for example, surrounded by a sleeve of insulating material which is again enclosed within a sleeve of metal. Consequently, even if splitting or chipping of the insulating material occurs, the insulating material will still be held in place by the sleeve of metal.

If it is desired to make the heater in the form of a hairpin in order to reduce its external magnetic field the following procedure may be adopted. The filament which is to constitute the heating member is, while straight, coated with two coats of kaolin in the manner hereinbefore described but the coating is only dried naturally or by heating and is not fired. The filament thus coated is then bent into the form of a hairpin with the legs touching and one or more further coats of kaolin are preferably applied as before described. The resulting unit may be then fired and may be coated with metal and oxides in the manner already described. In applying the sec- 0nd kaolin coat it is frequently found advantageous to spray this coat on the member, using a suspension of kaolin in water. This method of applying the coating may be of course employed in place of any of the other methods hereinbefore described. Moreover, the heating member may take any desired form and may be in the form of a straight filamentary conductor or it may consist of a helically wound filament in which case the two ends of the helix maybe welded to lead-in wires which preferably fit into the inside of the helix. Other forms of non-inductive heater than hairpin shaped may be used.

A thermionic valve employing a cathode prepared in the manner hereinbefore set forth may consist for example of an anode and grid supported in the usual-manner upon a pinch at one end of the envelope and registered at the other end by a glass bead. Two wires sealed into the pinch serve as lead-in wires for a hairpin shaped heater whilst another lead-in wire for the cathode also passes through the pinch. The upper end of the cathode may be spaced by means of a glass'bead. The end of the cathode nearest the pinch may have the lead wire welded to it, such wire passing through the pinch. Preferably the heater is held in resilient tension.

To enable the invention to be clearly understood and carried into effect the manufacture of various forms of electrode in accordance with the invention and the construction, also in accordance with the invention of a vacuum tube having an equipotential cathode, will now be described by way of example with reference to the accompanying drawings in which Fig. 1 illustrates on an enlarged scale an equipotential cathode whilst Fig. 2 is a greatly enlarged crosssection of the cathode shown in Fig. 1. Fig. 3 is a view similar to Fig. l of a cathode having a hairpin shaped non-inductive heater. Figs. 4, 5, and 6 are views illustrating the manufacture of a cathode with a non-inductive heater whilst Fig. 7 is a section on a greatly enlarged scale of the cathode illustrated in Fig. 3 or Fig. 6. Figs. 8 and 9 are views on an enlarged scale illustrating the manufacture of a cathode with a helical heater. Fig. 10 is a sectional view illustrating the moulding of a heater in the insulating material. Fig. 11 is a section on a large scale of the insulated heater moulded in the apparatus shown in Fig. 10. Figs. 12 and 13 are sections on a greatly enlarged scale of equipotential cathodes showing a method of applying the equipotential cathode surface. Figs. 14 to 18 inclusive are views illustrating a modified process of making an equipotential cathode with a hairpin heater. Fig. 19 is an elevation of a valve having an equipotential cathode in accordance with the invention, a portion of the anode being broken away, Fig. 20 being a perspective view illustrating on a larger scale the mounting of the electrodes, whilst Fig. 21 is a horizontal section of the electrodes shown in Fig. 19.

Referring first to Figs. 1 and 2 a tungsten wire I is provided with a coating 2 of the insulating material and an exterior metal casing 3, the coating 2 and the casing 3 being applied in any of the ways previously herein set forth. The metal casing 3 may be provided with an oxide coating to facilitate electron emission therefrom.

For the manufacture of an equipotential cathode with a hairpin-shaped non-inductive heater as shown in Fig. 3 a straight tungsten wire 4 (Fig. 4) is coated with the clay slip and dried but not fired. The wire is freed of insulation in the centre and at its two ends as shown in Fig. 4. It is then bent so that the partially coated portions of the wire are parallel and touching one another as shown in Fig. 5. The coated wire in this form is then dipped in the clay slip and receives a further coating which is dried and finally fired to sinter or vitrify the material. It will be understood that the final coating may be applied by spraying or otherwise in which case the heater may be mounted in a rotating turn table, care being taken to provide a coating of even thickness.- The metal casing may then be added in any of the'ways previously herein described, the resulting heater being shown in elevation in Fig. 3 and in section in Fig. 7. When the casing is applied by deposition of metal it will, as shown in Fig. '7, exactly conform to the configuration of the porcelain coating. In certain cases the firing may be done after the metal casing has been provided if the latter consists of a sufliciently refractory metal.

In Fig. 7 the coating 2 first applied and described with reference to Fig. 5 is indicated by the dotted circles whilst the finally applied coating is indicated at 2.

Referring to Figs. 8 and 9 the heater I is in the form of a helix the ends of which are welded to conductors 4 which may, for instance, consist of nickel wires extending into the ends of the helix. The helix is then coated, for instance, by dipping in clay slip, drying and firing in the manner previously herein described, a metal casing 3 being finally applied.

Referring to Figs. 10 and 11, 5 represents a mould and '6 its cover. A tungsten wire is partially coated in the manner describedin connection with Figs. 4 and 5 and is placed in the cavity I of the mould 5. Kaolin mixed with water to the consistency of a thick paste is placed in the cavity I and the cover 6 placed in position and forced down so that the final coating is moulded on. The heater will then have the form shown in Fig. 11. The mould and cover are preferably lubricated for example with colza oil in order to prevent the insulation sticking to them. The material thus moulded on the wire is then sintered or vitrified, the wire being preferably held under tension during this operation.

Referring to Figs. 12 and 13 the metal case in this instance comprises a metal tube, for in stance of nickel, which is formed by bending from sheet metal.

The sheet metal may be bent around a heater such as that shown in Fig. 5 or that shown in Fig. 6 or that shown in Fig. 11, or a tube may be separately constructed and the insulated heater slipped into it.

The modified process for the manufacture of a hairpin-shaped insulated heater illustrated by Figs. 14 to 17 inclusive is as follows. A tungsten wire I bent into the shape of a hairpin has welded to its ends a nickel member 8 which is of U or V- shape. The wire is then provided with an insulation coating by dipping in clay slip, care being preferably taken that the V-member 8 is not coated. The bight portion of the wire I has the insulation then removed from it (see Fig. 15) and the legs of the filament are then pressed together (Fig. 16). The insulation is then preferably fired whilst the tungsten wire is held under tension by means of a hook engaging its uncoated bight portion, the member 8 being also held. The heater thus constructed may then, for instance, be slipped into a nickel tube 3 (Fig. 17). The nickel tube 3 has a nickel or molybdenum wire 9 welded to it and is finally coated with oxides. When the metal casing is applied by a deposition process the wire 9 may be wound around the porcelain insulation and the metal casing deposited over both the insulation and the wire 9.

Although in the arrangements so far described the insulation is applied over substantially the whole of the heater wire, it will be understood that it need in fact only be applied over so much of itas is necessary to provide the requisite amount of insulation which may vary in different instances. An important advantage. obtained. by the invention is that an equipotential cathode adapted to be heated by means of alternating current can be; produced having a very small peripheral length. For instance, with a tungsten wire-heater of 5.1 mm. diameter, it has been possible to construct an equipotential cathode having a perimeter of 2 mm. Such a cathode when 4 cm. long-and provided withan oxide 'coating can be supplied with 1 ampere of alternating current at 4 volts, that-is to say, 5 watts .per square cm., and it is found that the heater has a long life. a

Referring now to Figs. 19, 20, and 21, the valve therein illustrated by way of example comprises a glass envelope Ill provided with a re-entrant tube II having at its upper end a pinch or seal I2 which carries lead-in wires and electrode supports, namely, an anode supporting wire I3, two heater supporting wires I4 and I5, a cathode lead-in wire I6 and a grid supporting member II, the latter being in the form of a socket.

The anode is indicated at I8, the grid at I9 and the equipotential cathode at 20. The grid consists of a tungsten helix secured to a grooved supporting member 2I as set forth in the specification of. Canadian Patent No. 259,416 dated30th March, 1926. The anode which is ofthe shape clearly shown in section in Fig. 21 is welded to the supporting wire I3.

The grid supporting wire 2I and the anode supporting wire I3 are tied together at their upper free ends. by means of Wires 22 and 23 respectively welded thereto, these wires being pinched into a glass bead 24. The cathode 20 is similar to that shown in Fig. 17, the V-shaped member 8 (Fig. 16) being cut through and its arms respectively welded to the support wires I4 and I5 whilstthe wire 9 is welded to the wire I5.

The hook 25 engages the exposed bight portion of the heater filament I said hook being welded to a leaf spring 2 which in turn is welded to a wire 21 the latter being secured in a bead 28 which also has'secured in it a wire 29 which is welded to the anode support I3. It will be seen therefore that the heater filament is held under tension and preferably when the cathode proper comprises a'separately formed metal tube it is free to slide' to a limited extent upon the insulated heater. I

It will be understood that various modifications may be made in the manufacture of the electrodes and in the electrode structures described without departing fromthe scope of the invention.

I claim as my invention:-

1. The steps in the production of an equipotential cathode of a thermionic discharge tube, which consist in applying a thin insulating coating on a thin heater filament by bringing into contact with the latter a refractory insulating material which is capable of being bonded by heat and is associated in finely divided form with liquid in the form of a sludge and fusing said material onto said thin heater filament by heat,

2. Steps in the production of an equipotential cathode of a thermionic discharge tube, which consist in dipping a thin heater filament into a sludge of paint-like consistency containing finely divided refractory insulating material which is capable of being bonded by heat, forming a thin substantially uniform coating, and then heating the filament whilst held in tension to a temperatu're'sufiicient to fuse the insulating material on to said thin heater'filament.

3. The method of making an equipotential electrode which consists in bending a straight thin filament to a substantially V-shape, attaching the-ends to a support, providing the legs of said filament with a thin coating of a semi-liquid sludge containing finely divided insulatingfmaterial, firing the filament, and surrounding the said filament'by a metallic cathode member.

4. The method of making an equipotential electrode,'which consists in bending a straight thin filament to a substantially V-shape, attaching the ends of a V-shaped metal support to the free ends of such V-shaped filament, providing the legs of said filament with a thin coating of a semi-liquid sludge containing finely divided insulating material, firing the filament, and surrounding the filament by a metal tube, and atany time after the firing, dividing the V-shaped support.

5. Steps in the production of an indirectly heated equipotential cathode of a thermionic discharge tube, which consist in-coating-a thin heater filament of highly refractory metal by bringing into contact with it a sludge comprising a liquid and a refractory insulating material which is capable of adhering in sludge form to said filament metal itself and of being bonded'to said filament by heat and is in a finely divided state, heating said insulating coating until it becomes fused on the heater filament, and then applying a thin cathode casing to said insulating coating.

6. The method of forming an equipotential cathode comprising coating a heat radiating electrically conducting body with a thermionically inactive dielectric coating, baking such coated body in a vacuum until fusionof said coating occurs, applying an electrically conducting coating to the outers'ide of the dielectric coating and heating such applied coating.

'7. An electron-emitting cathode comprising a metallic, equipotential cathode member and a heater, said heater comprising a filament, and an insulating, high heat-resisting coating fused on said filament.

8'. An electron-emitting cathode comprising a metallic cathode member and a solid heater, said heater comprising an electric conductor and a high heat-resisting insulating coating fused a ls-1am thereon and adhesively binding said conductor.

9. The process of manufacturing an equipotential electron-emittingcathode, said process including covering an-electric conductor with a substance, heating the said substance on the said conductor in a vacuum to form a refractory, insulating coating adhesively bound to said conductor, and subsequently surrounding the conductor, thus coated, by an electrically conducting and thermionically active cathode sleeve.

10. An indirectly heated electron-emitting cathode comprising'a metallic cathode member and a heater, said heater comprising an electric conductor and an insulating, high heat-resisting material ,atleast partially fused on said conductor.

11. An electron discharge device comprising a plurality of cooperating electrodes one of which comprises a metallic cathode member serving as an electron-emitter and a heater for heating said cathode member, said heater comprising a filament and an insulating, refractory material at least partially fused on said filament.

12. An electron discharge device comprising a plurality of cooperating electrodes, at least one of said electrodes being an electron-emitting cathode comprising a metallic sleeve serving as an electron-emitter and a heater for rendering said sleeve thermionically active and substantially enclosed therein, said heater comprising an electric conductor including a plurality of legs and an insulating, high heat-resisting adjunct formed on at least said legs and at least partially fused thereon.

13. The method of manufacturing an indirectly. heated electron-emitting cathode, said method comprising covering an electric conductor with a substance, subsequently fusing at least a portion of said substance to form a permanent, refractory, dielectric coating on said conductor, and applying on said so formed dielectric coating an outercoating of electrically conducting and thermionically active material, whereby a solid cathode is formed, having its components, including the heater, the dielectric coating, and the electron-emitting coating, adhesively bound to each other, thereby forming an integral, solid unit.

ERNEST- YEOMAN ROBINSON. 

